Answer:
a.invalid
b.invalid
c.invalid
d.invalid
Step-by-step explanation:
For option a return type of the function is not specified and there is no space between int and b.
For option b there is no space between int and ThisOne and char and x.If there is space then this is valid.
For option c data type of variable b is not specified.
For option d if YetAnother function is without arguments then parenthesis should be there parenthesis are missing in this function heading.